1. Field of the Invention
This invention relates to a system for testing electrical conductivity of electrical circuits; and more particularly, to a system that utilizes a non-contact plasma probe when testing electrical continuity and electrical shorts in circuit traces on printed wiring boards.
2. Description of the Prior Art
During the manufacturing process of printed wiring boards (PWBs) it is helpful to be able to test whether the printed circuit traces on the PWBs have electrical continuity from a starting point to a terminal point and are not shorted to adjacent circuit traces at any point along the printed circuit trace.
Conventional PWB testing means usually require a test probe to mechanically contact the PWB in order to perform the continuity test. Mechanical contact probes are inefficient because they require actual physical contact with each wiring trace that is to be tested. This process necessitates moving the conductive contact probes around to various locations on the printed wiring board. A probe physically moved by a preprogrammed X-Y coordinate mechanism is inefficient due to the number of rotations of either the probes or the PWB required to test the circuit traces. The "bed of nails" approach is also inefficient due to the number of physical contacts required by the probes on the PWB. These conventional mechanisms also have the drawback that as the size of components on the PWB have decreased the likelihood of damage to the PWB components, as a result of testing by contact probes has greatly increased. In an effort to avoid damage caused by probes impacting upon a PWB, non-contact probes have been developed. Present methods of non-contact testing include the following prior art patents.
U.S. Pat. No. 5,202,623 (LePage) discloses a laser-activated plasma chamber for non-contact testing. This reference discloses employing a laser-activated plasma in a metallic chamber that makes electrical connection to a node under test. This process utilizes a laser device to generate a plasma, which is then used to detect presence or absence of electrical current. This method requires a laser to generate the plasma and is inefficient because of the high energy requirements of the laser.
U.S. Pat. No. 5,179,279 (Millard et al.) discloses a non-contact electrical pathway. This patent discloses a method for establishing a signal pathway to and from a circuit through a conductive surface on the circuit by utilizing a laser beam to generate a plasma. The focused laser beam is directed towards a surface of a foil and forms a hole in the foil that yields a strand or a stream of plasma on the opposite side of the foil. A positioning mechanism is used to place a spot of the plasma on the surface of the board. The board is then laterally moved so that the spot of plasma can be selectively brought to the surface of conductive areas or paths, which represent circuit board connections. This method requires the use of a laser, which necessitates high energy consumption. This method further requires the additional apparatus of an optical system, a focusing lens and a foil to implement the system.
U.S. Pat. No. 5,268,645 (Prokoff et al.) discloses testing a circuit board by using an array of electrodes. A non-homogenous electric field is applied to the array and signals denoting the characteristics of the electric potential are generated.
As can be seen from the present state of the art, there exists a need for an improved method of testing conductivity of printed traces that does not require a laser. It would be desirable to utilize a single conductive stationary mechanically contacting probe and at least one non-contacting plasma probe such that the speed of the testing procedure would be increased and fragile elements on the PWB would not be damaged by moving a physically contacting probes about the PWB.